Sorghum is a genus of plants consisting of multiple species of grasses. Some Sorghum species are valuable crops, being used for food, animal feed and the production of alcohol and biofuels. For example Sorghum bicolor, commonly called sorghum, is one of the most important cereal crops for subsistence farmers in arid and semi-arid portions of Africa, Asia and the Americas. The crop is essential for human life on marginal lands throughout the poorest regions of the world. In developed countries, sorghum is important as a feed crop and as a crop that can be grown on marginal lands as part of a sustainable agroecosystem.
However, other Sorghum species are not so useful. For example, Sorghum halepense, commonly called Johnsongrass, is classified as an invasive species in the United States. This species is commonly found growing in crop fields and other open areas, such as pastures, and has been known to grow so quickly as to choke out the desired crops planted in farming fields. Another weed species in the Sorghum genus is S. propinquum. This species is closely related to Johnsongrass but is diploid, larger and has a different geographic distribution. Some studies have also suggested that Johnsongrass is a descendent of S. propinquum and possibly an interspecific hybrid descendent of sorghum and S. propinquum. (See, e.g., Paterson, et al., (1995) PNAS 92:6127; Paterson and Chandler, “Risk of Gene Flow from sorghum to Johnsongrass” (1996), available at http://www.isb.vt.edu/brarg/brasym96/paterson96.htm).
Unfortunately, Johnsongrass and sorghum are able to readily hybridize. While it is possible to phenotypically distinguish a hybrid plant from the cultivar, it is nearly impossible to distinguish between their seeds. This difficulty creates a genetic purity problem in sorghum seed lots. Because of this problem, one needs to be able to assess the level of Johnsongrass contamination in a seed lot to ensure that the seed lot is as pure as required or desired. Presently, such an assessment is commonly done through grow-outs, where 40,000 individual seeds are planted and grown and the number of hybrid plants counted. This solution is labor intensive, time consuming, expensive and inefficient.
Thus, there is a need for an improved method of detecting the presence of and/or quantifying the amount of Johnsongrass seed or Johnsongrass hybrid seed in sorghum seed lots.